This disclosure relates generally to systems and methods for detecting defects in wiring, and more particularly, to systems and methods for detecting partial discharges or arcing in wiring or cables.
Wiring is a critical system in aircraft, shipboard, industry and home applications. Aircraft wiring integrity and safety related issues are known to be serious and have received a great deal of interest after the Swissair 111 and TWA 800 accidents. Also, electrical fires in industry account for a large portion of property loss, and electrical fires in the home is a significant portion of the fires occurring in the home that threaten life and damage property.
Aircraft wiring insulation is much thinner than that found in building wiring in order to reduce weight. This thin insulation deteriorates with age due to changes in chemical composition, vibration during flights, large temperature changes, and exposure to agents such as dust, salt, moisture and cleaning chemicals. This wiring is also exposed to other mechanical stresses during maintenance. The aforementioned effects will degrade the insulation, causing cracks and chafing. These insulation defects can cause arcing between wires or surrounding metals. Humidity together with salt and dust depositions can make the arc creation more probable.
The detection of aircraft wiring defects is primarily performed by visual inspection by maintenance personnel. This manual inspection is a slow process and its reliability is not satisfactory. Furthermore, as it requires twisting the wiring in order to check chafing, this visual inspection often causes more problems than it can identify.
There is existing test equipment on the market for cable testing based on electrical measurements using Time Domain Reflectometry or Frequency Domain Reflectometry. Both Time Domain and Frequency Domain Reflectometry analyse signals produced by reflections from pulses, of predefined characteristics, transmitted through a wire under test. The sensitivity of these methods is usually not satisfactory to detect all insulation damage.
Time Domain Reflectometry is a testing and measurement technique that has found increasing usefulness in testing transmission lines (both metallic and fiber-optic), cables, strip lines, connectors, and other wideband systems or components. Time Domain Reflectometry is a technique in which reflections from a transmitted signal, e.g., an electrical pulse, are monitored to locate faults and to determine the characteristics of power transmission lines. The transmitted signal, preferably a very fast step pulse, is fed into the system and the reflections resulting from discontinuities or impedance deviations in the system are analyzed. When the input pulse meets with a discontinuity or impedance mismatch, the resultant reflections, appearing at the feed point, are compared in phase, time, and amplitude with the original pulse. By analyzing the magnitude, deviation, and shape of the reflected waveform, the nature of the impedance variation in the transmission system can be determined.
General Dynamics of Redmond, Wash. recently introduced a new test apparatus called the Micro-Energy Dielectric (MED) tool, as disclosed in U.S. Patent Application Publication No. U.S. 2002/0130668. This device uses a high DC voltage to generate discharges or arcs at insulation defects in a cable bundle under test. If a fault exists between the wire under test in the cable and any other grounded wires in the cable that has a breakdown voltage less than the maximum test voltage applied, the full discharge of the available charge stored in the cable, e.g., not a partial discharge, will occur at the fault, potentially further damaging the wire or cable. The location of the discharge is then determined in several ways by measuring the electromagnetic (e.g., RF region) and acoustic signals the discharge generates. First, the MED tool measures high frequency voltage pulse edges generated by the discharge at one end of the cable and determines the location of the discharge; second, a Electromagnetic Locating Tool (EML) measures the radiated (e.g., outside the cable) electromagnetic signal (radio waves) with receivers having suitable antennae and calculates the location of the discharge based on the arrival times of the signals at the receivers; and lastly, an Ultrasonic Locating Tool (ULT) measures the acoustic noise (e.g., sound waves) and the electromagnetic edge the discharge generates and determines the distance to the arc by timing the difference in arrival times of the two signals.
Therefore, a need exists for techniques to detect defects in wiring and cables that can be perform by means other than visual inspection and will not cause further degradation to the wire or cable under test.